Detection of Superheated Steam During Sterilization Using Biological Indicators

Saturated steam (SS) is used for sterilizing many medical devices. Exposure to SS for appropriate temperature/time combinations creates a microbicidal environment that renders product sterile. Superheated steam (SHS) has been heated beyond its saturation point and is less microbicidal, compromising process efficacy. Sterilization monitoring systems should detect SHS. One method is to use biological indicators (BIs; e.g., rapid-readout self-contained BIs [RRSCBIs]). The purpose of this study was to determine if RRSCBIs can detect SHS. Pressurizing the boiler to 4,700 mB, manifold to 4,000 mB, and chamber jacket to 3,600 mB and heating the viewing window to 150°C in a 10-L BI evaluation resistometer vessel allowed approximately 12°C and 4.5°C of superheat in a nominal 121.75 ± 0.25°C and 132.5 ± 0.25°C cycle, respectively, to be reproducibly achieved. Replicate tests using multiple RRSCBIs from different batches were exposed vertically (cap up), inverted (cap down), and horizontally to SS and SHS. RRSCBI viability was determined using a fluorescent readout method. RRSCBIs exposed to SS at 121.75 ± 0.25°C for 7 or 14 minutes were negative. A total of 135 type A RRSCBIs were exposed to SHS (12°C) at 121.75 ± 0.25°C for 14 minutes. Zero of 45 RRSCBIs mounted vertically showed a positive fluorescent result, 26 of 45 mounted inverted were positive, and 45 of 45 mounted horizontally were positive. A total of 135 type B RRSCBIs were exposed to SHS (12°C) at 121.75 ± 0.25°C for 7 minutes. Twenty-four of 45 mounted vertically were positive, 41 of 45 mounted inverted were positive, and 45 of 45 mounted horizontally were positive. RRSCBIs detected SHS, but this was orientation dependent. Further work is required to establish the application of these findings in healthcare facility settings.